FR2705166A1 - Battery separator and battery using it. - Google Patents

Abstract

The invention relates to a battery separator. According to the invention, it is composed of a microporous polyethylene membrane of ultra-high molecular weight, into which a monomer is grafted by radiation and it has the following properties: (a) a porosity of between approximately 50% and approximately 95%, (b) an average pore size of between about 0.1 and about 20 microns, (c) an electrolyte resistance of about 1 to about 50 mOMEGA-6.45 cm2, (d) maximum weight loss of 1% and a change in electrolytic resistance of not more than 25% after immersion in a 35% aqueous solution of KOH and 5% KMnO 4 at 50 ° C for 1 hour, (e) a tensile strength of between approximately 62.5 and about 98.2 kg / m in both length and width directions, (f) a KOH absorption ratio of about 5 to about 30, and (g) a permeability to Gurley air about 1 to 300 s / 10 ml; and (h) a thickness of about 12.7 mum to about 254 mum. The invention applies in particular to accumulator batteries.

Description

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  The present invention generally relates to batteries and more particularly, it relates to

  microporous separators for use in batteries.

  Batteries exist in many physical forms, employing various combinations of electrodes. With the advent of electric vehicles and other machines, the

  Demand for batteries with high energy density has increased.

  This demand had an impact on the design of the batteries.

  For such applications, it is desirable to make batteries that have electrodes very close to one another. However, when such batteries are built, there is an increasing risk that the battery will develop an internal short circuit due to the proximity of the electrodes. As an example of the problem mentioned above, one can

  refer to nickel-cadmium alkaline batteries.

  Traditionally, such batteries employ a sintered negative electrode. Nowadays, this type of electrode has been replaced by an electrode bonded to plastic in order to increase its storage capacity. However, when it is repeatedly charged and discharged, a battery having such an electrode may be much more likely to have an internal short circuit rather than such a battery having a sintered negative electrode. This problem of internal short-circuit is caused by a phenomenon known as "migration", where the active cadmium grows and is transferred

  from the negative electrode to the positive electrode.

  A method to overcome the above problem

  mentioned was to use microporous separators.

  The growth of active cadmium is delayed by a microporous separator and consequently this method makes it possible to substantially completely prevent short circuits.

  induced by the growth of active cadmium.

  However, such separators are problematic in that they have low gas permeation coefficients, creating the potential for undesirable rupture or venting of the battery. A second problem comes from

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  the inclusion of surfactants in such separators in order to obtain adequate wettability of the membrane. Such surfactants may escape from the separator, contaminate the environment of the battery and cause premature rupture and degradation of the battery.

drums.

  Wettable battery separators are disclosed in U.S. Patent No. 5,126,219 and these separators are made of ultra-high molecular weight, band-shaped, polyolefin fibers and fibers having a void volume. at least 20%. The wettability is imparted by incorporating a finely divided hygroscopic filler into the blend for extrusion of the polyolefin prior to extrusion. Such a formula imparts more permanent wettability to the material than is obtained by simple coating, but still poses problems

in actual use.

  There are many battery systems that exist

  and which require the use of a separator.

  Unfortunately, each system has its own conditions

  specific with respect to the properties of the separator.

  However, some properties of the separators are considered desirable, irrespective of the particular battery system to be used: (a) thin and reliable separation between the positive and negative electrodes, (b) very low electrolyte resistance in the electrolyte (c) long-term chemical stability when exposed to electrolyte and oxidants, even at elevated temperatures, (d) ability to absorb and retain a large amount of electrolyte, (d) good gas permeability , and (f) high mechanical strength, towards the

machine and transverse.

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  None of the marketed separators meet all

these criteria.

  Thus, there remains a need for a battery separator concurrently possessing a wide variety of desirable properties. The object of the present invention is to provide such a battery separator as well as a

battery containing it.

  The present invention relates to a battery separator which comprises a microporous polyethylene membrane of ultra-high molecular weight where is grafted by radiation a monomer, the separator having the following characteristics: (a) a porosity of between about 50% and about %, (b) an average pore size of from about 0.1 to about 20 microns, (c) an electrolyte resistance of from about 1 to about 50 mg2-6.45 cm2, (d) a maximum weight loss of 1% and an electrolytic resistance change of not more than 25% after immersion in an aqueous solution at 35% KOH and 5% KMnO4 at 50 ° C. for 1 hour, (e) a tensile strength of between about 62, 5 and about 98.2 kg / m, in both directions of length and width, (f) a KOH absorption ratio of about 5 to about 30, and (g) an air permeability from Gurley of about 1 to

300 s / 10 ml.

  The separator preferably has a thickness of about

12.7 pm to about 254 pm.

  The present invention furthermore relates to a battery comprising at least one pair of electrodes of opposite polarity, an electrolyte and a separator according to the present invention.

  invention placed between the electrodes of opposite polarity.

  The invention will be better understood and other purposes, features, details and advantages thereof

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  will become clearer during the description

  explanatory text which will follow with reference to the accompanying schematic drawing given by way of example only, illustrating an embodiment of the invention and in which: - the single figure is a scanning electron micrograph of a substrate useful for the preparation of the

  battery separator of the present invention.

  The present invention relates to a battery separator that has certain features that have not been concurrently available with the battery separators known to date. In particular, the separator of the present invention has a very low electrolytic resistance in a KOH electrolyte, which gives it a very high capacity. Moreover, the separator is permanently wettable and there is no surfactant or hygroscopic charge in the separator, which can leak from the separator when stored in an electrolyte and in water, even for long periods of time. periods of time. As a result, the electrolytic resistance does not change with time. The separators also have a high KOH absorption power, a high gas permeability and demonstrate symmetrical mechanical strength, without directional weakness. Moreover, the separators have excellent stability in a KOH electrolyte over a wide temperature range from boiling at -40 ° C, they have excellent oxidation stability and they undergo little weight loss when they are subjected to boiling KMnO4 / KOH. The

  Separators are also heat sealable.

  The separators of the present invention comprise an ultrahigh molecular weight ultrahigh molecular weight polyethylene microporous membrane which is grafted with monomer radiation, the separator having the following characteristics: (a) a porosity of between about 50% and about%,

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  (b) an average pore size of from about 0.1 to about 20 microns, (c) an electrolyte resistance of from about 1 to about 50 mΩ-6.45 cm 2, (d) a maximum weight loss of 1% and a change in electrolyte resistance of not more than 25% after immersion in an aqueous solution of 35% KOH and% KMnO4 at 50 ° C for 1 hour, (e) a tensile strength of between about 62.5 and about 98.2 kg / min, in both directions of length and width, (f) a KOH absorption ratio of between about 5 and about 30, and (g) an air permeability of Gurley from about 1 to

300 s / 10 ml.

  The separator preferably has a thickness of about 12.7 μm to about 254 μm. Advantageously, the separator does not shrink by more than about 2% in length and 1% in width after being kept at a temperature of 80 C for one hour, it has a CWST value of between about 0.072 and about 0.095 N / m and its resistance

  electrolyte is between about 5 and about 20 mΩ-

  6.45 cm2. Preferably, the value of CWST remains constant

  after extraction with boiling water for 30 minutes.

  The battery separator of the present invention is

  prepared according to the materials and processes described

  below. The starting material is a microporous polyethylene substrate which is made of high molecular weight polyethylene. The polyethylene must have an ultra high molecular weight ("UHMW"), i.e., a melt index at a standard load of less than about 0.04 for ten minutes and preferably 0, by measuring according to ASTM D 1238-70 and an intrinsic viscosity greater than about 3.0 (measured in decahydronaphthalene at 135 ° C). Preferred UHMW polyethylenes are those having a nominal weight average molecular weight of from about 500,000 to about 5 million.

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  by measuring according to ASTM D 4020-81. It is quite preferable to use the UHMW polyethylenes in the range of the highest molecular weights among those disclosed to be useful herein. Minor amounts of lower molecular weight polyolefins can be mixed therein. Such UHMW polyethylenes are known, for example, STAMYLAN U

(DSM, Geleen, Netherlands).

  The UHMW polyethylene microporous substrates useful in the present invention should have a stacked sheet-like structure in flakes with respect to the pores. The lamellar sheet-like structure can be viewed under a scanning electron microscope, as shown by the scanning electron micrograph of a useful substrate of Fig. 1, which shows a tortuous structure of the pores. In contrast, the upright pore structure of undesirable substrates such as some Celgard films is shown in scanning electron photomicrographs of the Celgard Technical Information-Film pamphlet (Celanese Corp., 1985). The substrate is preferably prepared by the gel extrusion process disclosed in European Patent No. 500173. Such a substrate is

  marketed under various grades by DSM (Geleen, Pays-

  Bottom) as hydrophobic battery separators

microporous UHMW-PE.

  The microporous substrate should have the following properties: a thickness of about 12.7 μm to about 254 μm, and preferably about 25.4 μm to 127 μm, a basis weight of about 3.0 to about 50 μm. g / m2, a tensile strength of about 62.5 to about 98.2 kg / m both in the machine direction (longitudinal) and transverse (width), a porosity of 50% to 95% and a size of

pores from 0.1 to 20 microns.

  The microporous polyethylene substrate is converted into a useful battery separator by making it hydrophilic permanently, without adversely affecting its original properties such as airflow, microporosity and

  mechanical strength, and the like.

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  Radiation grafting is the preferred technique for achieving such a result. The source of the radiation can come from radioactive isotopes such as cobalt-60, strontium-90 and cesium-137 or machines such as X-ray machines, electron accelerators and

ultraviolet equipment.

  The grafting method also includes the use of active monomers that will hydrophilize the polyethylene substrate. The selection of specific monomers will depend on the type of battery system. Monomers that are useful for grafting include all materials that are typically used for grafting polyethylenes, such as, for example, monomers that contain carboxylic groups. Examples of such monomers include acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, crotonic acid, sodium acrylate, methacrylate, and the like. sodium, potassium methacrylate, hydroethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate and hydroxypropyl methacrylate, hydroxy alkyl acrylates and hydroxy alkyl methacrylates. Preferred monomers are methacrylic acid and / or hydroxyethyl methacrylate. We can also

  use monomer mixtures above.

  The transplant will typically be obtained by one of the following processes. The substrate can be irradiated dry and

  subsequently soaked in a monomer solution.

  Alternatively, the substrate may be irradiated in the presence of a monomer solution. The monomer solution will comprise one or more of the monomers described above. The percentage of the monomers in the above solutions can be in general between 0.1% by weight and 100% by volume. Preferred transplant processes will be better described

in the examples that follow.

  The use of a microporous membrane which has the ability to remain wettable over a long period of time, by forming a battery separator, gives the

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  separator resulting in a greatly improved ability to absorb electrolyte compared with known microporous sheet separators. This property is particularly useful for forming batteries of the "electrolyte-poor" type (recombinant) where the ability of the separator to absorb and retain the electrolyte is critical in that the only electrolyte present is that absorbed by the separator and the plaque. The permanence of the hydrophilicity also allows a better performance

  and a longer life of the battery.

  The separators of the present invention have a wide variety of properties. The methods for evaluating a separator sample to determine if these properties exist are given in paragraphs

following.

  The process for determining the porosity of a separator involves the use of a Gurley Densitometer which measures the time it takes for a fixed volume of air to pass through the membrane at a given pressure. Of

  shorter times reflect more porous membranes.

  A more accurate measurement is to measure the density of the membrane (D) and the density of the polymer mass (Do) and to calculate the porosity using the formula:% porosity = Do Dx 100 Do The process for determining the average size pores of a separator is the bubble point technique using a Coulter porosimeter that gives the magnitudes

  maximum and minimum flow in the pores.

  The process for determining the electrolyte resistance of a separator is described in J. Cooper and others, editors "Characteristics of Separators for Alkaline Silver Oxide-Zinc Secondary Batteries" (Air Force Aeropropulsion Laboratory, Dayton, Ohio, September 1965), Chapter 6B . Samples of the separator to be tested are cut in triplicate and then quenched

  in an electrolyte at 40% KOH for one hour at 24 ° C.

  The cell used for the test is Pall Model 2401 RAI and consists of two half-cells with

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  Platinum electrodes operating at 1000 Hz. Measurements of cell resistance with the separator in place in the cell (Rs) and without the separator (Rc) in 40% KOH are recorded. The electrolytic resistance of the separator (R) is given by the equation: R = Rs-Rc The tensile strength of the separator is measured according to ASTM D-638-60T. An Instron table tester is used under the following conditions: 3.81 x 1.27 cm sample, 2.54 cm separation between clamps, cm / min strain rate, and 23 C temperature. tensile strength at break is calculated as F / W where F is the breaking load (kg) and W is the width of the sample (mm). The percentage of elongation is determined by subtracting the initial distance between the clamp from the elongation at break and dividing this result by the initial distance between the clamps. This result is multiplied by 100 to give a percentage of elongation. The absorption ratio of KOH and the absorption rate

  of KOH quantify the ability of the material to absorb KOH.

  To determine the absorption ratio of a sample, a sample of 0.2 to 0.5 g was weighed and placed in a beaker containing 200 ml of 40% KOH for one hour at room temperature. The sample is removed and left

  the excess electrolyte drip for three minutes.

  The wet sample is then reweighed. The absorption ratio is the weight of the sample when wet divided by the weight of the sample when it is dry. To determine the rate of absorption of a separator, a separator disk of 3.81 cm in diameter is placed in a beaker filled with 40% KOH. The time it takes to wet 90-100% of the surface of the sample is recorded. 90 seconds is the maximum time allowed for full saturation.

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  The oxygen stability or the oxidation resistance of a separator is important in that a separator which is not stable with respect to oxidation in a battery will result in poor electrical performance, manifested by a short shelf life and duration of use. The test used to measure oxidation resistance is described in J. Cooper et al., Editor "The Characteristics of Separators for Alkaline Silver Oxide-Zinc Secondary Batteries" (Air Force Aeropropulsion Laboratory, Dayton, Ohio, September 1965). The test requires placing about 1 g of the dry sample in a beaker containing 250 ml of 35% KOH and 5% KMnO4 and submerging it completely. With a glass lid, the beaker is kept at 50 ° C for one hour. The sample should then be washed in 5% oxalic acid followed by deionized water. The sample is subsequently dried and reweighed. Stability is complete by calculating the percentage of weight loss and changes in electrolyte resistance. The percentage of weight loss is given by the formula:% weight loss = Wi - Wf x 100, where w is the initial weight and Wf is the final weight of the sample. The long-term high temperature stability of a separator is a parameter that can be determined by examining changes in size and electrolytic strength of a sample after soaking the sample in KOH at 80 ° C for 24 hours. All major changes in these properties are an indication of degradation. To determine long-term stability, a sample of the 11.4 x 3.81 cm separator was tested for initial electrolytic strength in 40% KOH. the sample is then immersed in 40% of KOH and kept at 80 ° C. for 24 hours. After cooling, the dimensions of the sample and its electrolytic resistance are measured. Any change of dimension is

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  calculated as a percentage of expansion. This percentage is determined by subtracting the area of the sample after soaking from the area of origin, dividing this result by the area of origin of the sample and multiplying by

100.

  The air permeability of a separator is a measure of the time required for a fixed volume of air to pass through a standard area under a slight pressure. The process is described in ASTM D-726-58. The instrument used for this test is a Gurley Model 4110 densitometer. To carry out the test, a sample is inserted and fixed in the densitometer. The cylinder gradient is raised to the dc line (100 ml) and then dropped under its own weight. The time (in seconds) it takes for the passage of

  100 cc of air through the sample is recorded.

  The thickness of a sample is the average of ten thickness measurements taken on a 929 cm2 section of the sample. The critical wetting surface tension (CWST) of a porous medium is the surface tension between that of the liquid that is absorbed and that of the liquid that is not

  absorbed, in a predetermined time, by the porous medium.

  Thus, liquids with lower surface tensions than CWST of a porous medium will spontaneously wet the medium upon contact and if the medium is porous, will easily pass through it. On the other hand, liquids with a greater surface tension than CWST of a porous medium may not pass through at all with small pressure differences and pressure differences.

  high enough, they can cross irregularly.

  As disclosed in U.S. Patent No. 4,880,548, the CWST value of a porous medium can be determined by individually applying drops of a series of liquids having surface tensions ranging from 0.002 to 0.004 N / m. and observing the absorption or non-absorption

of each liquid with time.

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  A rewettability test was designed to measure the wettability of the separator after being extracted with a solvent such as water or methylene chloride. If the wettability is not permanent, the CWST value decreases and the electrolyte resistance increases rapidly. To undertake this test, the CWST value and the electrolytic resistance of a sample of the separator are measured and recorded. An 11.4 x 3.8 cm piece of this sample is cut out and immersed in water at 80 ° C. for 30 minutes. After cooling and further rinsing, the piece is air-dried and CWST and electrolytic resistance are measured. The CWST and the electrolytic resistance are then compared to the values obtained from a test of the original sample. A second piece of 11.4 x 3.8 cm is cut from the same separator sample and is immersed in methylene chloride for 15 minutes at room temperature. The piece is then washed with deionized water and air dried. The value of CWST and the electrolytic resistance are then measured and compared with the

  obtained test values on the original sample.

  The separators of the present invention can replace the separators commonly used in a wide variety of batteries including both batteries.

  flooded as alkaline batteries poor in electrolyte.

  Separators can also be used in a wide variety of other batteries such as nickel-cadmium, nickel-hydride metal, nickel-zinc batteries,

  nickel-iron, zinc-air, silver-cadmium, zinc-

  manganese dioxide and zinc-halogen (such as zinc-chlorine and zinc-bromine). The design and construction methods of the above batteries are well known to those who are

competent in the matter.

  The examples which follow will better illustrate the present invention but of course, should not limit the

frame.

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Example 1

  This example illustrates the characteristics of the microporous polyethylene membranes of the type useful in the preparation of the battery separator of the present invention. Four microporous polyethylene membranes having varying properties were obtained from DSM under the trade designation UHMW-PE Microporous Hydrophobic Separators. Each of the membranes has been subjected to a wide variety of tests and

  characteristics are shown in Table 1 below.

Table 1

  Properties Substrate 1A Substrate 1B Substrate 1C Substrate 1D Thickness (gm) 20 17.8 20 30 Base weight (g / m2) 8.59 6.55 3.9 7.26 Traction (kg / m) DM = 67.8 55.4 28.57 30.36

D.T. = 73.2 83.9 58.9 73.2

  Lengthening (%) DM = 291 391 446.5 410.8

D.T. = 389 317.9 267.9 214.3

  CWST (N / m) 0.030 0.030 0.033 0.030 KL alcohol (bars) 0.17 0.89 0.41-0, 0.138-0

  Melting point 140.5 142.8 143.4 -

(C, DSC)

  Stability at 140.5 142.8 143.4 -

  heat (% shrinkage) C / 4 h 16.60 29.69 89 25 C / 8 h 4.63 6.29 21 12 C / 4 h 1, 56 8.96 20 13 C / 16 h 1.56 1, 75 10 6

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Example 2

  Two of the microporous polyethylene membranes of Example 1, i.e., substrates 1A and 1B, are passed through a monomer bath consisting of 6% methacrylic acid, 10% hydroxyethyl methacrylate, 05% diethylene glycol dimethacrylate, 25% TBA and 58.95% deionized water at a rate of 9.1 meters / min. The membranes instantly became transparent. Wet membranes were exposed to 10 Mrad of electron beam radiation. After exposure, the membranes were heated in an oven at 60 ° C for 1 hour. The membranes were then rinsed in deionized water for 2 hours and dried in an oven where air was circulated at 60 ° C. for 20 minutes. The separators, respectively designated 2A and 2B, were wettable with water, having a CWST value of 0.090 N / n, and had a very low electrolytic resistance of 9 mΩ-6.45 cm 2. As demonstrated in Example 1, the non-grafted membranes were hydrophobic having a CWST value of about 0.030 kg / m and had a very high

electrolytic resistance.

  Other features of the present separators A and B are shown in Table 2 as well as, for comparison, the characteristics of a commercial separator, Celgard

3401 (Hoechst-Celanese).

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Table 2

  Properties Example 2A Example 2B CelgardO 3401 Thickness (ym) 38 25 25 Base weight (g / m2) 10.87 7.75 16.87 Tensile strength (kg / m)

DM 80.37 80.37 342.9

DT 71.44 82.15 32.14

  Electrolytic resistance (mQ2-6.45cm2 in 40% KOH at room temperature) CWST (N / m) 0.090 <5sec 0.090 <lmin 0.090 <min CWST after soaking in MeCl2 for 15 minutes 0.086 <10sec 0.086 <10sec 0.032 Gurley air flow 55 306 (s / 10ml) 6 KOH absorption rate 6 8 6 (s) Absorption ratio of KOH 13.5 22.5 2.91 (wet g / g dry) Dilatation in KOH (%) Room temperature LOOO

1 0 0 0

 C L +0.6 +0.5 0.5

1 +0.8 +0.8 +0.8

  Thermal stability (%) when dry: 80 C 1 hour -1,3 -1, 3 -2,5 wet in KOH: 80 C +1,4 +1,3 +1,3 1 hour

Long-term stability (half

  6.45 cm2 at 80 C / 24h) Initial resistance 9 8 21 Final strength 9 6 14

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  As can be seen, samples 2A and 2B are very thin and have a lower basis weight than Celgard 3401. The samples do not exhibit asymmetrical tensile strengths in the machine and transverse directions. They have a much higher gas permeability and absorb much more of the KOH electrolyte than Celgard 3401. These properties are very

  important for poor secondary batteries.

Example 3

  The microporous polyethylene membrane of Example 1, i.e., the 1C substrate, was wound by interfolizing Reemay and quenched in a monomer solution composed of 0.8% methacrylic acid, 0.5% hydroxyethyl methacrylate, 20% TBA, 1% polyethylene glycol dimethacrylate (600) and 77.7% deionized water. The

  roll was evacuated for 15 minutes and sealed.

  Subsequently, the roll was exposed to gamma radiation at a dose of 10,000 rad / hr for 20 hours to a total dose of 0.2 Mrad. The roll was unrolled and rinsed in deionized water for 2 hours. It was then dried in an oven where air was circulated at 60 ° C. for 20 minutes. The resulting separator was wettable with water and had a thickness of 0.203 μm and an electrolytic resistance of 36 ml-6.45 cm 2. This sample shows that the hydrophilicity can be obtained by gamma radiation grafting. The dimensional stability of the separator in different

  electrolytes is shown in Table 3.

Table 3

  Electrolyte Dilatation dilated in length width

% KOH 0% 0%

28% NaOH 0% 3%

% H2SO4 5.5% 0%

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Example 4

  A microporous polyethylene membrane of Example 1, i.e., substrate 1C, was passed through a monomer bath comprising 10% methacrylic acid, 1.5% polyethylene glycol 600 dimethacrylate, % TBA and 58.5% deionized water at 9.1 m / min. The membrane immediately became transparent. The wet membrane was then exposed to radiation from an electron beam to a total dose of 10 Mrad. The resulting separator was rinsed in deionized water for 1 hour and dried in an oven where air was circulated at 60 ° C. for 20 minutes. The resulting separator was wettable with water and had a

  thickness of 23 μm and an electrolytic resistance of 14 minutes

  6.45 cm2. Electrolytic resistance at high temperatures

  of the separator is shown in Table 4.

Table 4.

  Temperature Electrolytic resistance (mQ-6.45 cm2)

 C 17

 C 9

 C 5

 C 6

 C 6

  The separator is relatively stable in the KOH electrolyte at temperatures between 40 and C and furthermore has a very low electrolyte resistance.

Example 5

  A microporous polyethylene membrane of Example 1, i.e., the 1D substrate, was exposed to electron beam radiation under an inert nitrogen atmosphere to a total dose of 10. Mrad and then immersed in a monomer bath consisting of 10% MA, 30% TBA and 60% distilled water. The grafted membrane was left

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  at rest for 48 hours at room temperature and then rinsed in deionized water for 2 hours and air dried overnight. The separator was wettable with water, having a CWST value of 0.085 N / m. The separator was wetted in 15 seconds with a% KOH solution and instantly wetted with a 40% concentrated H2SO4 solution. The separator had good uniformity

  and had an electrolytic resistance of 10 mΩ-6.45 cm 2.

  The separator was compatible after 24 hours at 50 C both in the KOH electrolyte and in concentrated H 2 SO 4.

Example 6

  60 cm of the polyethylene membrane of Example 1, i.e., the 1D substrate, was wound into a roll with interfolded paper. This roll was then introduced into a test tube which contained a 10% aqueous solution of an acrylic acid monomer. The tube was evacuated with a vacuum pump for 5 minutes and sealed. The tube and its contents were irradiated in a Co-60 pit at a dose of 10,000 rad / h for 20 hours. After the radiation treatment, the grafted membrane was washed with hot water and dried. The resulting separator membrane was wettable with water and had an electrolyte resistance of 25 mΩ-6.45 cm 2. This example shows that acids other than methacrylic acid can be used for

  graft to obtain hydrophilic separators.

Example 7

  The procedure of Example 6 was repeated except that an aqueous solution of 10% methacrylic acid monomer was used instead of the 10% acrylic acid monomer aqueous solution. The resulting separator was wettable with water and had an electrolytic strength of 12 mΩ-6.45 cm 2 in 40% KOH and 18 mΩ-6.45 cm 2 in 40% H 2 SO 4. The separator was stable in both electrolytes of KOH and H2SO4. After a week in KOH and H2SO4, the separator had a resistance

  electrolyte of 10 mI-6.45 cm 2 in 40% KOH and 27 nm -1.

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  6.45 cm2 in 40% H2SO4. The physical integrity of the separator

remained intact.

Example 8.

  A roll of a microporous polyethylene membrane of Example 1, i.e. substrate 1B, was exposed to an electron beam under an inert nitrogen atmosphere (oxygen concentration lower than 40). ppm) up to a total dose of 10 Mrad. The membrane was then immediately passed through a bath of a solution containing 10% hydroxyethyl methacrylate, 6% methacrylic acid, 0.05% diethylene glycol dimethacrylate, 25% TBA and 58.95. % deionized water. The roll was stored under nitrogen for 4 days at room temperature. It was subsequently washed with dripping for 4 hours and air dried. The resulting separator is instantly wettable with water and exhibited excellent uniformity of electrolytic resistance over its entire width and length. A further characterization of this separator is given in Table 5 as well as a comparison with the separator

Celgard 3401 Trade.

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Table 5

  Properties Celgard 3401 Example 8 Microporous microporous film type Thickness (μm) 0.025 0.046 Base weight (g / m 2) 16.9 13.5 Electrolytic resistance 9 (min-6.45 cm 2 in 35%

KOH at 23 C)

  Absorption (%) 35% KOH Blotting method 80 290 Gould method (%) 220 650 (drainage 2 minutes) Dilatation (%) in 35% KOH

L +0.1 +0.5

W +0.4 +1.2

  350 130 Gurley air flow (s / 10 ml) Tensile strength (kg / m)

DM 342.9 80.37

DT 32.14 82.15

Elongation at break

DM 44 19.4

DT 536 16.0

  Stability in KOH (%) in 35% KOH for one hour boiling (weight loss) Dilatation (%) in air C for 1 hour L -3.6 -8

1 0 0

 C for 1 hour L -4.6 -0.9

1 0 0

  Heat Sealability YES Heat As can be seen, the separator of the present invention has a lower electrolytic resistance than Celgard 3401. It also absorbs a larger amount of the KOH electrolyte and has a higher gas permeability. This example also demonstrates another way of electron beam substrate grafting to prepare a

  separator according to the present invention.

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Example 9

  This example demonstrates the higher electrical resistance at very low temperatures achieved with the separators of the present invention. Example 8 demonstrated that the present separator had a lower electrical resistance at room temperature than Celgard 3401 which is a well-known commercial microporous battery separator. The lower resistance offered by the separator of the present invention makes it possible to charge and discharge a battery more quickly, which is a very desirable property. The electrical resistance of the present separator and those marketed has been further compared to low temperatures. At temperatures below zero, the electrical resistance of the present separator has increased much more slowly with decreasing temperature compared to the Celgard 3401 separator. Therefore, at low temperatures, the batteries using the separators of the present invention will have higher power and higher power capacity than conventional separator batteries. The electrical resistance of the present separator of Example 8 and the Celgard 3401 separator to various

  temperatures is given in Table 6.

Table 6

  Separator 250C -10 C -20 C -30 C -40 C Example 8 9mn-6,45cm2 26 44 86 158 Celgard 20 56 85 155 310

Example 10

  This example demonstrates the superior stability to oxidation possessed by the separators of the present invention.

invention at high temperatures.

  A sample taken from the separator prepared in Example 8 was cut into a 50 x 100 mm strip and heated in a beaker containing 5% KMnO4 / 35% KOH.

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  for one hour at 50 C. A similar sample was taken from a Celgard 3401 separator and treated similarly. Both samples were washed with 5% oxalic acid to dissolve any residual KMnO4 product was rinsed with deionized water. The percentage of weight loss, the initial electrical resistance and the final electrical resistance were determined before and after exposure of the two samples and the results of these tests are given in

Table 7.

Table 7

  Separator% Loss of Resistance Resistance final initial weight (mQ-6.45 cm2) (mQ-6.45 cm2) Example 8 0.15 9 7 Celgard 3401 15 20 >> 5000 * average of three separate tests These results demonstrate that Batteries using separators of the present invention will have a greater duration of use than conventional separators at exposures under the same conditions. The Celgard 3401 separator had a much greater weight loss and became substantially non-conductive under conditions that did not significantly affect the separator of the

present invention.

Example 11

  This example demonstrates the permanence of hydrophilicity

  possesses the separator of the present invention.

  Unlike other separators which obtain their hydrophilicity by addition of wetting agents or hygroscopic agents in the membrane, which agents can leak from the separator, rendering it hydrophobic again, the separator of the present invention has a hydrophilicity which does not is not susceptible to degradation by

  leak. Two tests have been done to illustrate this fact.

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The first test consisted in extracting a sample with water at 80 ° C. for 30 minutes and the other test consisted of extracting the sample with methylene chloride at room temperature for 15 minutes. The CWST and electrical resistance values for the samples of the present invention and the Celgard 3401 separators were

  determined and the results are shown in Table 8.

Table 8

  CWST CWST separator after CWST after Resistance Initial resistance initial extraction extraction after (N / m) water (N / m) to MeCl4 (mf2extraction (N / m) 6.45cm2) with MeCl2 (ma- 6.45cm2) Example 2A 0.090 0.090 <Ssec 0.086 9 Example 2B 0.090 0.090 <5sec 0.086 8 10 Celgard 0.090 0, 090> 5sec 0.032 22> 5000 0.087 10sec It is readily apparent that the Celgard 3401 separator loses its hydrophilicity and becomes hydrophobic during exposure to an organic solvent such as methylene chloride. Moreover, its electrolytic resistance rises strongly. On the contrary, there is no remarkable change in the present separator of the invention in a

  exposure similar to an organic solvent.

Example 12.

  This example demonstrates the superiority of the separator of the present invention in the field of performance

  electrochemical in high temperature cycle conditions-

low temperature.

  An electrochemical cell was constructed with a zinc negative plate and a silver oxide positive plate using the separator prepared in Example 8 as well as the Celgard 3401 separator.

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  electrolyte, a 40% aqueous solution of KOH. The open circuit voltage was measured and recorded. The cell was heated at 60 ° C. for 30 minutes and then cooled at 5 ° C. for 75 minutes. After heating to room temperature, the open circuit voltage was measured and recorded a second time. This cycle was repeated a number of times with the monitored open circuit voltage as in the first cycle. After the cycles, the

  cell was disassembled and the separators were examined.

  As shown in the results shown in Table 9, the open circuit voltage for the cell using the separator of the present invention did not drop as fast as that employing the Celgard 3401 separators in such hot-cold cycles. Use of the separator of the present invention has leveled at a voltage of about 1.50 volts without falling below 1.10 volts. Battery voltage using the Celgard 3401 separator fell well below 1.50 volts

to below 1.10 volts.

  This result can be explained by a set of effects.

  First, the silver oxide plate of the battery with the Celgard 3401 separator is reduced faster to a white color, thus oxidizing the wetting agents in the separator to a darker color. Secondly, the expansion and contraction of the electrolyte helps the leakage of wetting agents that are not permanently bonded to the Celgard 3401 separator. Third, a portion of the silver oxide on the surfaces of the plate oxidizes the agents. wetting agents of the Celgard 3401 separator, causing

thus a fall in the voltage.

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  Table 9 Circuit voltage Example 8 Celgard 3401 open (volts) Initial 1.69 1.70 Warm cycle 1 1.69 1.68 Cool cycle 1 1.59 1.42 Warm cycle 2 1, 59 1.45 Cool cycle 2 1.55 1.25 Warm Cycle 3 1.56 1.28 Cold Cycle 3 1.52 1.10 Warm Cycle 4 1.54 1.15 Cold Cycle 4 1.52 1.06 One Night at Temperature 1.69 1 , 70 Ambient Warm Cycle 5 1.69 1.68 Cool Cycle 5 1.59 1.42 Warm Cycle 6 1.59 1.45 Cool Cycle 6 1.55 1.27 Warm Cycle 7 1.56 1.28 Cool Cycle 7 1.52 1.10 Hot cycle 8 1.54 1.15 Cold cycle 8 1.52 1.06 Disassembled cell Separator light brown very dark brown Plate Ag20 some parts mainly black white All references cited here are incorporated in reference title.

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Claims (23)

  1. Battery separator, characterized in that it consists of a microporous membrane made of a polyethylene of ultra-high molecular weight is grafted by radiation a monomer, and in that it has the following properties: a porosity of from about 50% to about%, (b) an average pore size of from about 0.1 to about 20 microns, (c) an electrolyte resistance of from about 1 to about 50 mf2-6.45 cm 2, (d) a maximum weight loss of 1% and a change in electrolyte resistance of not more than 25% after immersion in 35% aqueous KOH and 5% KMnO4 at 50 ° C for 1 hour, (e) a tensile strength of between about 62.5 and about 98.2 kg / m in both directions of length and width, (f) a KOH absorption ratio of about 5 to about 30, and g) a Gurley air permeability of about 1 to 300 sec / 10 ml; and   (h) a thickness of about 12.7 μm to about 254 μm.   2. Separator according to claim 1, characterized in that it does not shrink more than about 2% in length and 1% in width after being maintained at a temperature of  C for an hour.   3. Separator according to claim 2, characterized in that it has a critical wetting surface tension value of between about 0.072 N / m and about 0.095 N / m.   4. Separator according to claim 3, characterized in that its critical wetting surface tension value remains substantially constant after extraction with water boiling for 30 minutes. 27 2705166   5. Separator according to claim 3, characterized in that its electrolytic resistance is between about 5 and 20 Qm-6.45cm2.   Separator according to claim 1, characterized in that the monomer is selected from the group consisting of   monomers having a carboxylic acid group.   Separator according to claim 6, characterized in that the monomer is selected from the group consisting of acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, crotonic acid, sodium acrylate, sodium methacrylate, potassium methacrylate, hydroxyalkyl acrylates,   hydroxyalkyl methacrylates and mixtures thereof.   8. Separator according to claim 6, characterized in that the monomer is selected from the group consisting of acrylic acid, methacrylic acid, hydroxyethyl methacrylate, hydroxypropyl methacrylate, acrylate   hydroxyethyl acrylate, hydroxypropyl acrylate and mixtures thereof.   9. Battery, characterized in that it comprises at least one pair of electrodes of opposite polarity, an electrolyte and a separator according to claim 1, placed   between the electrodes of opposite polarity.   10. Battery according to claim 9, characterized in   what it is a stale nickel-cadmium battery.   11. Battery according to claim 9, characterized in   what it is a nickel-metal hydride battery stale.   12. Battery according to claim 9, characterized in   what it is a stale nickel-zinc battery.   13. Battery according to claim 9, characterized in   what it is a stale nickel-iron battery.   14. Battery according to claim 9, characterized in   what is a zinc-air battery.   15. Battery according to claim 9, characterized in that   what it is a silver-cadmium battery.   16. Battery according to claim 9, characterized in   what it is a zinc-manganese dioxide battery. 28 2705166   17. Battery according to claim 9, characterized in   what it is a zinc-halogen battery.   18. Battery separator according to claim 1, characterized in that the aforementioned membrane has a tortuous structure of the pores. 19. Separator according to claim 18, characterized in that the membrane has a slat sheet structure stacked with respect to the pores.   Separator according to claim 19, characterized in that the monomer is selected from the group consisting of   monomers having a carboxylic acid group.   Separator according to claim 20, characterized in that the monomer is selected from the group consisting of acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, crotonic acid, sodium acrylate, sodium methacrylate, potassium methacrylate, hydroxyalkyl acrylate,   hydroxyalkyl methacrylates and mixtures thereof.   22. Separator according to claim 20, characterized in that the monomer is selected from the group consisting of acrylic acid, methacrylic acid, hydroxyethyl methacrylate, hydropropyl methacrylate, acrylate   hydroxyethyl acrylate, hydroxypropyl acrylate and mixtures thereof.   23. Battery, characterized in that it comprises a pair of electrodes of opposite polarity, an electrolyte and a separator according to claim 19 placed between the electrodes of opposite polarity.